1. Field of the Invention
This invention relates in general to containers for hauling bulk products and in particular to a dump truck or trailer with the capability of dumping conventionally by gravity or conveying upward to a storage facility.
2. Description of the Prior Art
In oil and gas well hydraulic fracturing treatments, a liquid under very high pressure is pumped into the well in order to cause the formation to fracture. While the fractures remain open due to the high pressure being exerted, granules of sand or other small hard particles are mixed with the liquid being pumped. Granules enter the fractures, propping them open when the pressure is relieved. To inject the sand, a large mixing machine is used to mix the sand with water and chemicals. Due to the high flow rate, the sand must be dumped in a high rate. Conventional trailers or trucks having tilting beds dump directly into the hopper of the mixer in order to achieve this high flow rate.
Various sizes or grades of fine sand are used in the fracturing process. Typically, the sand is purchased at a remote area, then shipped to a terminal by rail. At the rail terminal, the sand is conveyed to a pneumatic or pressure tank truck, which then hauls the sand to a field office of the service company that handles fracturing treatments. Air pressure is applied to the tank to blow the sand to a silo or storage facility at the field office.
When a well is to be treated, the sand may be handled in one or two ways. For large jobs, a temporary storage facility or silo may be set up at the well site. Sand from the field office silo or the rail terminal is hauled by a pressure tank truck to the well site. Air pressure is used to blow the sand from the pressure tank into the temporary silo. During the fracturing process, sand from the temporary silo is conveyed to a dump truck, which in turn moves over and dumps directly into the mixer. Known conveyors of reasonable size do not have sufficient capacity to handle the high flow rate required for dumping sand in the mixer. Consequently, the pressure tank truck is not able to discharge directly into the mixer hopper.
In smaller jobs, a well site silo may not be used. Dumps trucks may haul the sand directly from the field office to the site. Both methods have disadvantages. In the first case, two types of units are required at the well site, a pressure tank truck and a dump truck. In the latter case, only a dump truck is needed at the well site, but a pressure tank truck is still required to haul the sand from the rail terminal to the field office in most cases. Also, in both cases, if only a part of the load of sand in the dump truck is needed, often the rest must be discarded by dumping on the ground. There is no provision for conveying the sand from the dump truck to a pressure tank truck or to a silo.
A truck or trailer that has the capacity to dump by gravity out the rear into a hopper as well as the capacity to convey up to a storage facility would in many cases eliminate the need for a pressure tank unit. Also wastage of sand left over in the dump truck could be avoided if the dump truck had the capability of conveying the sand to a storage facility. If would be particularly desirable to accomplish the conveying without the need for making the dump truck into a pressure tank. Pressure tanks must be essentially air tight and capable of withstanding up to 25 pounds per square inch pressure, although operable usually at 12 pounds per square per inch pressure. This requires heavy construction and makes the unit more expensive.
U.S. Pat. No. 3,099,497, Albert, discloses a pneumatic conveyor and tank that is mounted on wheels and tilts to facilitate conveying. It, however, requires a pressure tank. Also it is not possible to dump by gravity into a hopper with the device disclosed in this patent.